The present invention relates to a radio-frequency power amplifier device used in a radio transmitter, and in particular to a radio-frequency power amplifier device in which the output power is automatically controlled to be constant.
FIG. 4 is a block diagram showing a prior-art radio-frequency power amplifier device for receiving and amplifying a radio-frequency input signal 1 (hereafter referred to as RF input signal). As illustrated, it comprises a radio-frequency power amplifer 2 having a gain control terminal 3 for varying the gain of the power amplifier 2 and producing an amplified radio-frequency output signal (hereinafter referred to as RF output signal) 4, a coupling capacitor 5 for extracting part of the RF output signal 4, a detector circuit 6 comprised of diodes and a smoothing circuit for detecting the radio-frequency signal and producing a detected voltage output 7, a reference voltage setting circuit 8 for outputting a reference voltage 8a corresponding to the power level that has been set in accordance with a power level switch control signal 10a supplied from a controller 10 and used for switching the output power, and a comparator/low-pass filter circuit 9 comparing the detected voltage 7 with the reference voltage 8a and having a low-pass filter (LPF) characteristics.
In operation, the RF input signal 1 is power-amplified by the power amplifier 2 to become the RF output signal 4. Part of the RF output signal 4 is input to the detector circuit 6 via the coupling capacitor 5. The detector circuit 6 performs the peak-detection, and outputs the detected voltage 7 corresponding to the magnitude of the RF output signal 4. The comparator/LPF circuit 9 compares the detected voltage 7 with the reference voltage 8a to detect their difference or error between them, and passes the error signal through the LPF to smooth it, and then outputs it. The error signal 9a output from the comparator/LPF circuit 9 is input to the gain control terminal 3 provided on the power amplifier 3.
The path from the coupling capacitor 5 through the comparator/LPF circuit 9 to the gain control terminal 3 forms a negative feedback circuit. When the RF output signal 4 becomes larger than the target value, the detected voltage 7 increases and a negative error signal is created to reduce the gain of the power amplifier 2. When the RF output signal 4 becomes smaller, a positive error signal is created, and the gain of the power amplifier 2 is increased. In this way, the RF output power is automatically controlled to be constant.
The reference voltage setting circuit 8 has a power level switching input terminal for permitting the target value to be set from the controller 10. By such setting, different set voltages can be output. Accordingly, any one of a plurality of output power values (power levels), from a low-power output to a high-power output, that is set by means of the controller 10, can be selected.
Because the prior-art radio-frequency power amplifier device is configured as described above, when the output power (power level) is switched, the magnitude of the signal input to the detector circuit 6 also varies. Accordingly, the detector circuit 6 must have a wide dynamic range to cope with all the power levels. As a result, when a low-power output is set, the detected voltage 7 is minute, and the temperature variation of the forward-voltage of the detector diodes forming the detector circuit 6 gives a large influence on the output power value.